The present invention relates to a keyboard unit that is employed as an input unit for an information processor and a key switch incorporated into this keyboard unit. More particularly, the present invention relates to a keyboard unit and a key switch suitable for thin portable personal computers.
FIG. 1 is an exploded perspective view schematically showing a conventional key switch. FIGS. 2A and 2B are vertical sectional views showing the key switch of FIG. 1. FIG. 2A shows when the key switch is not depressed and FIG. 2B shows when the key switch is depressed by an operator.
As shown in the figures, the key switch 1 is provided on a base plate 2 made of metal or plastic or the like. The key switch 1 has a membrane sheet 3 with an electric contact 3a, and a click rubber 4 consisting of an elastic insulator which is provided on the membrane sheet 3.
The click rubber 4 has an elastic dome portion 5 covering the electric contact 3a within the membrane sheet 3, and a sheet portion 6 provided on the membrane sheet 3.
Also, the key switch 1 has a key cap 7 which is in contact with a top portion of the dome portion 5, and a key cap cover 8.
The key cap 7 has a ceiling plate 7a, a support shaft 7b and a brim portion 7c. The key cap cover 8 has bearing portions 8a which support the support shaft 7b of the key cap 7 in such a way that the key cap 7 can be swung on the bearing portions 8a, and an edge portion 8b which comes in contact with the brim portion 7c of the key cap 7 when the key cap 7 is not depressed. The edge portion 8b of the key cap cover 8 has a function as a stopper for preventing the key cap 7 from slipping out upward.
In the key switch 1 with the above-described structure, as shown in FIG. 2B, when the key cap 7 is depressed by the operator, it will be rotated downward on the support shaft 7b and the dome portion 5 will be buckled and elastically deformed. The deformation of the dome portion 5 causes the protrusion 5a of the inner surface of the dome portion 5 to press against the electric contact 3a within the membrane sheet 3.
In the key switch 1, incidentally, in order to prevent the finger of the operator from striking on the key cap cover 8 and reducing the operability when the key cap 7 is depressed, it is necessary in the free state shown in FIG. 2A (i.e., when the keycap is not depressed) that the length H.sub.1 from the upper surface of the key cap cover 8 to the uppermost portion of the key cap 7 be made longer than the key stroke H.sub.S between the protrusion 5a and the electric contact 3a. It is also necessary that the length H.sub.2 from the lower surface of the base plate 2 to the upper surface of the key cap cover 8 be made longer than the length Hc from the lower surface of the base plate 2 to the top portion of the dome portion 5.
In the key switch 1, however, supposing the key stroke H.sub.S is lengthened, the position of the upper surface of the edge portion 8b of the key cap cover 8 which is utilized as a stopper is higher (that is, the length H.sub.2 will be higher). For this reason, the length H.sub.1 has to be made longer and the dimension H.sub.0 (=H.sub.1 +H.sub.2) from the lower surface of the base plate 2 to the uppermost portion of the key cap 7 becomes longer, so that there arises the problem that the keyboard unit becomes thick. Also, conversely, in order to achieve thinning of the keyboard unit, the keystroke H.sub.S must be sacrificed. In other words, lengthening of the stroke of the key switch 1 and thinning of the keyboard unit are incompatible to each other.